A field emission display (FED) has been studied and developed as a next-generation high-luminance flat panel display. An incandescent lamp and a fluorescent lamp have been used over the years as a light emission device for general illumination, and a fluorescent lamp has such characteristics that the electric power consumption thereof may be suppressed as compared to an incandescent lamp for the same luminosity, and is being widely used as illumination. In late years, a display device and illumination using a light emitting diode (LED) as a light source are developed as a substitute of the ordinary illumination, such as an incandescent lamp and a fluorescent lamp, and are being popularized. Most recently, they are utilized in a display device, such as a traffic light, a backlight for LCD, various illumination devices, and the like.
LED functions by the theory of light emission caused by recombination of the carriers of the semiconductor, and thus emits monochromatic light having an inherent wavelength that is determined by the band structure of the material, and furthermore LED is a point light source. Accordingly, LED is not suitable particularly for an application to uniform emission in a large area such as a backlight or illumination, or light having a broad wavelength range, such as white light. In the case of display in white, particularly, such a constitution is necessarily employed that LED is used as an ultraviolet ray emission device, and a fluorescent material is made to emit light with the ultraviolet ray.
It may be considered on the other hand that a thin and high-luminance surface light emission device is obtained easily by making a fluorescent material to emit light with electrons emitted from a surface electron emission source by the similar process as FED.
In field emission type electron emission source (i.e., a field emitter), when the intensity of the electric field applied to a substance is increased, the width of the energy barrier on the surface of the substance is gradually narrowed corresponding to the intensity of the electric field, and under an intense electric field having an electric field intensity of 107 V/cm or more, electrons in the substance may run through the energy barrier by the tunnel effect. The phenomenon that the substance emits electrons accordingly is thus utilized. In this case, the electric field is in accordance with Poisson equation, and thus cold electrons may be efficiently emitted with a relatively low extraction voltage by forming a portion where the electric field is concentrated in the member that emits electrons (i.e., the emitter).
In recent years, carbon nanotubes (which may be hereinafter referred to as CNT) are receiving attention as an emitter material. CNT is a hollow cylinder formed by rolling up a graphene sheet having carbon atoms regularly arranged, and the outer diameter thereof is in a nanometer order, whereas the length thereof is generally from 0.5 μm to several tens micrometer, which provides a considerably high aspect ratio. CNT is expected to have a high electron emission capability since the electric field is liable to be concentrated thereon due to the shape thereof. CNT also has such features as high chemical and physical stability, and thus it is expected that CNT is hard to receive influence of adsorption of the residual gas, ion impact and the like in vacuum where the emitter is operated.
As a production method of an electron emission source using CNT, a method of coating a dispersion liquid containing CNT on a substrate, followed by drying and baking, is considered to be excellent in productivity and production cost, and thus has been studied variously.
CNT is in the form of very fine fibrous fine particles (powder), and in the case where an electron emission source is formed by using CNT, it is necessary to fix CNT to a substrate. In general, a binder material, such as a resin, is used for fixing CNT. Specifically, a binder material and CNT are mixed with and dispersed in a solvent to form a paste (or an ink), which is coated on the surface of the substrate by such a measure as a printing method, a spraying method, or a die coater method, followed by drying and baking, so as to fix CNT onto the substrate by utilizing the adhesiveness of the binder material. In the case where CNT is fixed onto the substrate by this method, CNT itself is in the state where it is embedded in the binder material, and accordingly, for realizing high electron emission characteristics, such a method has been employed that CNT is exposed, and CNT is arranged perpendicular to the substrate. For example, PTL 1 describes the technique, in which a porous sheet member having adhesiveness is adhered to a surface of a layer containing CNT, followed by drying, and then the sheet member is peeled off, thereby exposing CNT and arranging CNT perpendicularly. Furthermore, PTL 2 describes the technique, in which a layer containing CNT is dry-etched. Moreover, as a method for exposing CNT present inside a film, PTL 3 proposes the method, in which a film, which has been formed by coating a composition containing CNT, an oligomer, a crosslinkable monomer, a polymerization initiating material and a solvent on a substrate, is subjected to a heat treatment to form cracks in the film with thermal stress, and thus CNT is exposed inside the cracks, thereby providing an electron emission source.